Entropy-Enabled Site Inversion and Defect Activation in Medium-Entropy Spinels for Near-Black Infrared Emission

Spinel infrared (IR) coatings are attractive for high-temperature thermal management but often deliver sub-black emissivity and suffer thermal drift. Although entropy-stabilized oxides have been explored, most prior demonstrations remain case-by-case without a unifying principle for tailoring infrared radiation. Here, we report a medium-entropy inverse spinel, where configurational entropy drives partial site inversion and defect formation, coherently tuning both band structure and lattice dynamics. The ceramic achieves near-blackbody emissivity (∼0.90, 0.78-16 µm) and more than doubles the parent oxide in the 2-8 µm band critical for radiative heat transfer above 1000°C. It remains a single-phase and higher emissivity after 200 h at 1300°C. The material exhibits low thermal conductivity (0.53 W·m^-1·K^-1, 800°C), and as a sprayable coating reaches a hemispherical emissivity of ∼0.96 on steel and refractories, increasing furnace temperature by ∼44.3°C. Spectroscopy and density functional theory (DFT) reveal entropy-driven mixed-valence states, abundant oxygen vacancies, band gap narrowing (∼0.7 eV), and activation of IR phonons. This work establishes a generalizable strategy, coupling configurational entropy and site inversion, to convert robust spinels into near-black, durable radiators, offering a scalable platform for energy-saving coatings in extreme environments.